For information storage in electronic devices, at present DRAMs, which are semiconductor memories, are mainly used. However, they require refresh cycles at certain periods due to their gradual loss of memory from leakage of current. Further, they consume electric power, presenting environmental problems. Thus, attention is being paid to ferroelectric random access memories, which are nonvolatile memories (with no memory loss) not requiring refresh cycles.
Ferroelectric random access memory is derived from DRAM by substituting its capacitor portion consisting of silicon oxide with a ferroelectric thin film and utilizes a phenomenon in which spontaneous polarization of the ferroelectric thin film can be switched between two different remnant polarization states by applying an electric field. Because the state of the remnant polarization is maintained after removal of the electric field, it makes up a nonvolatile memory. Besides being nonvolatile in memory and capable of random access, a ferroelectric random access memory is provided with merits such as lower write voltage, higher-speed write capability, less consumption of electric power, a greater number of write-erase cycles and higher compatibility with DRAM than EEPROM and flash memories, which are conventional nonvolatile memories. Ferroelectric random access memories possess the most advantageous characteristics for information storage among the presently known nonvolatile memories.
Ferroelectric raw materials for use in ferroelectric random access memories require characteristics such as large amounts of polarization (switched charge amounts), small dielectric constants, good resistance to polarization fatigue, good memory retention, high speed in polarization switching, and low leakage currents.
A representative example of ferroelectric materials that have been used for ferroelectric random access memories is PZT (a solid solution of lead titanate and lead zirconate represented by the formula Pb(Zr, Ti)O.sub.3), which has a large amount of polarization and a relatively small dielectric constant. Although SrBi.sub.2 Ta.sub.2 O.sub.3 (SBT) having excellent polarization fatigue characteristics is known, it suffers from the drawback that it requires a high processing temperature exceeding 700.degree. C. and has a small amount of polarization.
Generally, methods for forming oxide thin films of composite are classified as vapor phase methods such as sputtering and CVD, and liquid phase methods such as the sol-gel processing. A PZT thin film can be formed by one of these methods. The vapor phase methods can form uniform films, but devices therefor are costly and generally suffer from low productivity. Additionally, in the CVD method the film composition is likely to vary, thus also causing the problem that the film characteristics are unstable. For sol-gel processing, it is easy to control the film composition, but difficult to form a uniform film because the film tends to powder. In this regard, Japanese Patent Laid Open No. 4-19911 describes that where the thin films of PZT or the like are formed by the sol-gel processing, uniform thin films can be obtained if a stabilizer such as a .beta.-diketone is added to coating solutions.
PZT is very advantageous as a ferroelectric raw material for use in a nonvolatile memory in that it has a large amount of polarization which is the most fundamental requirement for ferroelectric random access memory, or nonvolatile ferroelectric memory. However, thin films made of such materials experience the problems that they are susceptible to polarization fatigue (a decrease in the amount of polarization as the result of having been subjected to repeated polarization inversions) and their resistance to the polarization fatigue (also simply referred to as "fatigue characteristic") is insufficient. A poor fatigue characteristic shortens the life of a RAM, which results in limited utility.
Another problem with a PZT is that its memory retention and imprint characteristics are insufficient. The retention characteristic is that which reflects the degree of retention of the data written into a memory in the operating environment. The imprint characteristic is that which shows the degree of deterioration of a ferroelectric thin film in the operating environment by virtue of voltage stress, pulse, polarization inversion, temperature or the like. These characteristics also heavily influence the usable life of a ferroelectric random access memory.